Mentioning Brahmin makes a good point. Can make bio disel with left over brahmin parts and other bio- fuel from crap.
Still one train, one driver a few mechanics to move thousands of troops is way cheaper then. Trucks which you'll need a driver for each one. Mechanics to rebuild and maintain those trucks. The Fuel for those trucks, parts, the infrastructure needed to move thousands of troops.
Its not really a matter of can they do it? Its a matter of which is cheaper and easier to do.
Also nuclear power is not really an option. World running low on uranium before the war.
I hope the Engine of FO4 allows for wheels because FO1 and FO2 have large Caravans of Brahmin pulling carts made out of old cars. I want to see that again insted of brahmin carring everying.
Good points there.
Though I just tittered you realise with the release of FO1+ that made the apocalypse 100% man made.
Now you have mass herds of mutated two headed cows as a source for food, transport, clothing and just about everything else in the wastes.
Current evidence showing that livestock has a mass contribution to climate change make the FO universe doomed from the start by a unseen enemy.
Keep an eye out on Skyrim Styles as that's likely to give you a clue for a future FO..
Though if not in at the start if the engine is capable of it a modder will figure a way to do it.